Photovoltaic solar cells, including crystalline silicon solar cells, may be categorized as front-contact or back-contact cells based on the locations of the two polarities of the solar cell metal electrodes (emitter and base electrodes). Conventional front-contact cells have emitter electrode contacts on the cell frontside, also called the sunny side or light capturing side, and base electrode contacts on the cell backside (or base electrodes on the cell frontside and emitter electrodes on the cell backside in the case of front-contact/back-junction solar cells)—in either case, the emitter and base electrodes are positioned on opposite sides of the solar cell. Back-contact cells, however, have both polarities of the metal electrodes with contacts on the cell backside. Major advantages of back-contact solar cells include:                (1) No optical shading and optical reflection losses from the metal contacts on the cell sunny side, due to the absence of metal electrode grids on the front side, which leads to an increased short-circuit current density (Jsc) of the back-contact solar cell;        (2) The electrode width and thickness may be increased and optimized without optical shading concerns since both metal electrodes are placed on the cell backside, therefore the series resistance of the emitter and base metal grids are reduced and the overall current carrying capability of metallization and the resulting cell conversion efficiency is increased;        (3) Back-contact solar cells are more aesthetically appealing than the front-contact cell due to the absence of the front metal grids.        
Solar cell structure and fabrication methods, particularly for back-contact mono-crystalline silicon solar cells utilizing thin silicon substrates, may be found in related U.S. Pat. Pub. Nos. 2012/0305063 published Dec. 6, 2012, 2013/0233378 published Sep. 12, 2013, and 2013/0167915 published Jul. 4, 2013, all of which are hereby incorporated by reference in their entirety. In U.S. Pub. No. 2013/0233378, the thin silicon substrate is a standard czochralski (CZ) wafer with a thickness reduced by mechanical surface grinding or chemical silicon etching (or another method such as cleaving thin silicon substrates from thicker wafers using proton implantation or stress induced cleavage). In U.S. Pub. Nos. 2012/0305063 and 2013/0167915, the thin silicon substrate is an epitaxial-grown thin film silicon substrate (TFSS). Here, the epitaxial silicon layer may be initially grown on a porous silicon release layer on top of a reusable silicon template and then released/separated from the template at the porous silicon release layer after a partial or full completion of the cell fabrication process steps. Both the thin CZ wafer and TFSS may be substantially planar or consist of regular or irregular three-dimensional micro-structures.
However, there are challenges associated with back-contact solar cells, which include:
(1) Due to the relatively thinner substrate thickness (in the range of about 1 μm to 100 μm, and less than 50 μm in some embodiments) the substrate must be mechanically supported and reinforced with a more rigid back plane/plate during processing in order to prevent cracking of the thin silicon and resulting manufacturing yield losses; and
(2) The co-planar interconnections of the metal electrodes require higher electrode positioning accuracy than front-contact solar cells in order to prevent fatal shunting between the counter electrodes attaching to the base and emitter regions.
Designing cell architecture and manufacturing processes to prevent these and other problems associated with back contact solar cells remains a challenge as obtaining a high manufacturing yield of back contact solar cells requires robust fabrication processes and an effective cell design.